The divalent cation, Ca ion, serves as a central regulatory signal for numerous cellular processes particularly in excitable tissues. Studies in this laboratory have shown that an acidic Ca ion binding protein present in high concentration in neurosecretory tissues is structurally related to the calcium binding subunit of skeletal muscle troponin (TN-C). This TN-C like protein (TCLP) is identical to the calcium dependent activator of cyclic nucleotide phosphodiesterase and adenyl cyclase studies by other investigators. Studies of the structure, function, and evolution of this important regulatory protein currently in progress in our laboratory include: 1) determination of the complete amino acid sequence of bovine brain TCLP; 2) comparison of the structure and function of brain TCLPs isolated from a number of vertebrate species; 3) determination of the ability of TCLP to function as a regulator of the Ca ion sensitive ATPase activity of muscle and non-muscle actomyosins; 4) elucidation of the nature of the recognition sites on TCLP responsible for its interaction with adenyl cyclase, cyclic nucleotide phosphodiesterase and subunits of the actin relaxing complex. Studies of the biological function of TCLP are in progress directed toward elucidating the role of this protein in regulating the movement, metabolism, and growth of eucaryotic cells. BIBLIOGRAPHIC REFERENCES: Herman, A. C. and Vanaman, T. C. (1975). Micro Analytical and Preparative Peptide Separations with a Modified Technicon Autoanalyzer. Analytical Biochemistry, 64: 550-555. Vanaman, T. C., Harrelson, W. G. and Watterson, D. M. (1975). Studies on a Troponin-C Like Ca ion Binding Protein from Brain, Fed. Proc. 34: 307.